Circuit arrangement for starting detection of three-phase generator

ABSTRACT

In a circuit arrangement for measuring a starting of rotation of a generator having an exciting winding and three-phase windings, a voltage regulator regulates an output voltage of the generator by influencing an exciting current, the three-phase windings are connected to one another at a point, one of the phase windings is connected to ground via a first resistor, another of the phase windings is connected to ground via a second resistor, a voltage across the first resistor is evaluated as a rotational-speed-dependent alternating voltage, this voltage is compared with a threshold voltage, and the starting of the rotation of the generator is detected when the measured voltage exceeds the threshold voltage.

BACKGROUND OF THE INVENTION

The present invention relates to a circuit arrangement for measuring therotational speed of a generator. More particularly, it relates to acircuit arrangement for a starting detection of a three-phase generator.

In motor vehicles, three-phase generators which excite themselves afterthe motor driving them has started up are usually used to generate theelectrical energy. So that this self-excitation occurs reliably andsafely, measures which improve the self-excitation are usually initiateddirectly after the motor vehicle engine starts.

One customary method consists in using the charge monitoring lamp topre-excite the generator. For this purpose, during starting, energy isfed to the exciter winding from the battery via the charge monitoringlamp. Another method consists in providing an additional lead betweenthe battery and the exciter winding of the generator and conducting anadditional exciter current via this connection during the startingprocess.

The aforesaid measures are disclosed, for example, in the GermanOffenlegungsschrift 38 43 161 and the corresponding U.S. Pat. No.5,107,198. In the known solutions, additional means for detecting thestart of the engine are required.

However, in generator voltage controller systems, it is also known touse the rotational-speed-dependent alternating voltage in a phasewinding of the three-phase generator in order to detect starting. Insuch a case, a voltage measurement is carried out in the voltagecontroller, during which voltage measurement the voltage is measuredbetween the phase and earth and, when a threshold value is exceeded, itis detected that the engine is starting and thus the three-phasegenerator is beginning to rotate.

This type of starting detection has the disadvantage that, if theswitch-on threshold is too high, the cut-in speed of the generator istoo high and, if the threshold is too low, the signal-to-noise ratiobecomes too small, so that starting may be detected erroneously.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide acircuit arrangement of the above mentioned general type, which avoidsthe disadvantages of the prior art.

More particularly, it is an object of the present invention to provide acircuit arrangement of the above mentioned type, which has the advantagethat either the threshold voltage can be increased when the cut-in speedis the same, as a result of which the signal-to-noise ratio is improved,or that the cut-in speed can be lowered. The threshold voltage here isthe voltage which, when exceeded, permits the start of rotation of thegenerator to be detected.

This advantage is achieved in that the voltage between one phase andearth is evaluated in a generator with star-connected phase windings,while a different phase winding is connected to earth via a furtherresistor. By means of this measure, a voltage divider is obtained whichconsists of the input resistor of the voltage meter and the additionalresistor, this voltage divider being connected to the phase-to-phasevoltage between the two phase windings. This phase-to-phase voltage ishigher than the phase voltage by a factor √3.

Furthermore, it is advantageous that the threshold voltage which, whenexceeded, permits the start of rotation of the generator to be detected,can be set easily by suitable selection or variation of the furtherresistor.

The novel features which are considered as characteristic for theinvention are set forth in particular in the appended claims. Theinvention itself, however, both as to its construction and its method ofoperation, together with additional objects and advantages thereof, willbe best understood from the following description of specificembodiments when read in connection with the accompanying drawings. FIG.1 shows a circuit arrangement according to the invention, and in FIG. 2an equivalent circuit diagram is illustrated which permits therelationships to be more easily recognized.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a three-phase generator G with the phase windings U,W and V which are connected to one another at a common center point MP.The phase windings U, V and W are connected to the diodes or Zenerdiodes of the rectifier bridge GL via connections u, v, w.

The rectifier diodes GL are connected via connections B+ to the positivepole of the battery and via B- to the negative pole of the battery B orearth. In each case a further diode or Zener diode D7, D8 can beconnected between the center point MP between the phase windings and B+or B-. The diodes of the rectifier bridge are designated by D1 to D6.

The phase windings u, v, w have voltages Uu, Uv, Uw. The voltage Uu ispresent between the connection u of the winding u as shown in FIG. 1.The voltage U_(u) is present between the connection u and the ground asshown in FIG. 1.

In order to measure the rotational speed n_(G) of the generator, theresistor R1 across which the voltage U1 drops is connected between theconnection v of the phase winding V and earth. This resistor R1 is theinternal resistor or the input resistor of a voltage detection device S.

The rotational speed of the generator G can be determined from thevoltage U1, which is a rotational-speed-dependent alternating voltage,in that for example the period of the alternating voltage is determinedand the rotational speed is calculated from it in a known manner. Byevaluating the level of the voltage U1 and comparing it with a thresholdvoltage US, a rotation of the generator can be detected when the voltageU1 exceeds the voltage US.

An additional resistor R2 across which the voltage U2 drops is connectedbetween the connection w and earth. This resistor substantially improvesthe voltage evaluation at R1, so that the detection of the rotation ofthe generator is also improved.

The evaluations are usually also carried out in the voltage regulator R.

In FIG. 2 an equivalent circuit diagram with the voltage sources Uv andUw is illustrated, the said diagram indicating the voltage division in acircuit with the resistor R2. In this arrangement, the two voltagesources replace the phase windings V and W with the voltages Uv and Uw.

If a generator phase which is different from that used for detection ofthe rotational speed in the regulator is connected to earth via aresistor R2, a voltage divider consisting of R2 and the input resistorR1 of the voltage-detection and the rotational-speed-detection circuitof the voltage regulator is connected to the phase-to-phase voltage Uvwof the generator in the arrangement selected in the exemplaryembodiment. This voltage is higher than the phase voltage by the factor√3.

Since the voltage to be evaluated is greater than one of the phasevoltages by the factor √3, in comparison with rotational-speedmeasurement using the evaluation of a phase voltage, there is a greatervoltage range which can be utilized to determine the start of rotationof the generator.

Thus, the voltage limit value which, when exceeded, permits rotation ofthe generator to be detected can be increased in the voltage range whichis made higher by √3, as a result of which the signal-to-noise ratio isenlarged and the risk of rotation of the generator being erroneouslydetected is reduced.

However, it is also possible to operate with a relatively low limitvalue; as a result of the voltage which is increased by √3, incomparison with the evaluation with a phase voltage alone, rotation ofthe generator is detected earlier and the excitation can be initiated atan earlier point; the so-called cut-in speed of the generator is thenlower in comparison with evaluation using one of the phase voltageswithout the additional resistor according to the invention.

By varying the value of R2, any desired voltage to earth can be set atthe fixed input resistor R1 of the voltage regulator. The thresholdvoltage at R1 can thus be reduced by means of R2, so that the rotationof the generator is detected by the voltage regulator at lowerrotational speeds of the generator when the measured voltage exceeds thethreshold voltage.

After it has been detected that the generator is rotating, the start ofthe internal combustion engine is also detected and the voltage regularR can directly initiate measures which bring about an improvement inpre-excitation.

Reference E identifies an exciting winding of the generator whilereference I_(E) identifies an exciter current which flows through theexciting winding and controls the voltage regulator R so that theinitial voltage of the generator increases to a desired value. Thesegenerator components are known from the prior art and described forexample in U.S. Pat. No. 5,107,198.

It will be understood that each of the elements described above, or twoor more together, may also find a useful application in other types ofconstructions differing from the types described above.

While the invention has been illustrated and described as embodied in acircuit arrangement for starting detection of three-phase generator, itis not intended to be limited to the details shown, since variousmodifications and structural changes may be made without departing inany way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can, by applying current knowledge,readily adapt it for various applications without omitting featuresthat, from the standpoint of prior art, fairly constitute essentialcharacteristics of the generic or specific aspects of this invention.

What is claimed as new and desired to be protected by Letters Patent isset forth in the appended claims.

What is claimed is:
 1. A circuit arrangement for a starting detection ofa generator having an excitor winding and three-phase windings,comprising a voltage regulator which regulates an output voltage of thegenerator by influencing an exciting current of the excitor winding;means for connecting the three-phase windings of the generator at apoint; a first resistor arranged so that one of the phase windings isconnected to ground via said first resistor; a second resistor arrangedso that another of the phase windings is connected to ground via saidsecond resistor; means for evaluating a voltage across said firstresistor as a rotational-speed-dependent alternating voltage; means forcomparing said rotational-speed-dependent alternating voltage with athreshold voltage; and means for detecting a beginning of the rotationof the generator when the evaluated voltage exceeds the thresholdvoltage.
 2. A circuit arrangement as defined in claim 1, wherein saidsecond resistor is variable so that a predetermined voltage to ground isset at said first resistor.
 3. A circuit arrangement as defined in claim1, wherein said voltage regulator is formed so that after it has beendetected that the generator is rotating, said voltage regulatorinitiates an excitation.